The invention relates to a distance measuring apparatus comprising a photoelectric light receiver and a photoelectric light transmitter which is preferably arranged alongside the photoelectric light receiver, which alternately transmits high frequency modulated light wave trains to the light receiver via a measurement path and a reflective target arranged at its end, wherein the distance of the target from the light transmitter/light receiver is to be measured, and via a reference path of known length provided between the light transmitter and the light receiver, preferably within the housing; and also an electronic evaluation circuit which preferably contains a microprocessor and which evaluates the light wave trains which are alternately received by the light receiver with respect to the difference of the transit time via the measurement path, on the one hand, and the transit time via the reference path, on the other hand, and which deduces from this the spacing of the light transmitter/light receiver from the target, taking account of the speed of light and the known length of the reference path.
Such pieces of distance measuring apparatus (DE-OS 34 29 062) serve, for example, to measure by electronic means distances which lie in a range from 2 to 10 m from the distance measuring apparatus, with the measurement accuracy lying in the range of a few mm. In this way the filling levels of containers and also for example the degree of filling of shelves can be detected from a technological measuring viewpoint and transmitted to a central station by an electronic route.
Such distance measuring apparatus normally use a measurement receiver and a reference receiver which involves the danger of differential drifting of the two receiver circuits and leads moreover to disadvantageous effects of different depletion layer capacities or reaction time changes of the two photodiodes used for reception. On the other hand, the use of only one light transmitting diode brings the problem that cross-talk between the measurement and reference paths cannot be precluded.